Certain diacetylenes are known to be capable of forming colour on exposure to light. 10,12-Pentacosadiynoic acid is well known in the art to be an example of such a diacetylene. This compound is initially colourless in its unreacted state, but on exposure to UV light undergoes a topochemical polymerization reaction to generate a blue coloured polydiacetylene, which can then be transformed into a red coloured form by thermal perturbations.
WO06/018640 teaches the application of diacetylenes, such as 10,12-pentacosadiynoic acid, in multi-colour printing applications in combination with a photoacid or photobase generating species. Colour forming diacetylenes, such as 10,12-pentacosadiynoic acid and the like, are typically very reactive, and can undergo the initial polymerization reaction on exposure to fluence values as low as 50 mJcm−2. The consequence of this high reactivity is poor stability to background radiation. Light sensitive diacetylenes will gradually polymerise and turn blue on storage, even in the dark as often high light sensitivity is combined with high thermal sensitivity. In order to generate colourless coatings with these compounds it is usually necessary to purify them, via re-crystallisation, prior to use, which is time consuming and wasteful. Also, any coatings made using these diacetylenes will gradually turn blue on exposure to background radiation. This severely limits the range of applications that the coating can be used in.
WO 2009/081385 describes a thermoplastic material comprising a polymer and at least one polychromic substance. The polychromic substance is a functionalized diacetylene. In the Examples, 10,12-Pentacosadiynoic acid is used. The materials may be subject to some discolouration upon prolonged storage in the dark.